Singh admitted in a statement Wednesday that he used deer antler spray but said he didn't know it contained a substance -- IGF-1 -- banned under the PGA Tour's Anti-Doping Policy.
PGA Tour players were told in 2011 that deer antler spray use was implicated in positive drug tests. Singh hasn't failed a drug test.
"I am absolutely shocked that deer antler spray may contain a banned substance and am angry that I have put myself in this position," Singh said in his statement.
Singh, 49, cited a back injury as the reason he was withdrawing from the Phoenix Open. He is still listed in the field for next week's AT&T Pebble Beach National Pro-Am in California.
He was one of the athletes from several sports implicated in a Sports Illustrated article about a company called S.W.A.T.S. -- Sports with Alternative to Steroids. The article alleged that Singh paid $9,000 for S.W.A.T.S. for deer antler spray and other products.
Among the other athletes mentioned in the Sports Illustrated article is Baltimore Ravens linebacker Ray Lewis, who is to retire after Sunday's Super Bowl. Lewis referred to the allegations as "the work of the devil."
IGF-1 stands for insulin-like growth factor-1. It is believed to improves a user's endurance among other alleged benefits.
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